(function($) {
    $.fn.lazyload = function(options) {
        var settings = {
            threshold : 0,
            failurelimit : 0,
            event : "scroll",
            effect : "show",
            container : window,
            original : "original"
        };
        if (options) {
            $.extend(settings, options);
        }
        /* Fire one scroll event per scroll. Not one scroll event per image. */
        var elements = this;
        if ("scroll" == settings.event) {
            $(settings.container).bind("scroll", function(event) {
                var counter = 0;
                elements.each(function() {
                    if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) {
                        /* Nothing. */
                    } else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) {
                        $(this).trigger("appear");
                    } else {
                        if (counter++ > settings.failurelimit) {
                            return false;
                        }
                    }
                });
                /* Remove image from array so it is not looped next time. */
                var temp = $.grep(elements, function(element) {
                    return !element.loaded;
                });
                elements = $(temp);
            });
        }
        this.each(function() {
            var self = this;
            /* 触发事件显示图片 */
            $(self).one("appear", function() {
                if (!this.loaded) {
                    $("<img />").bind("load",
                            function() {
                                $(self).hide().attr("src", $(self).attr(settings.original))[settings.effect](settings.effectspeed);
                                self.loaded = true;
                            }).attr("src", $(self).attr("original"));
                }
                ;
            });
            /* 指定触发事件 */
            if ("scroll" != settings.event) {
                $(self).bind(settings.event, function(event) {
                    if (!self.loaded) {
                        $(self).trigger("appear");
                    }
                });
            }
        });
        /* 指定容器触发事件 */
        $(settings.container).trigger(settings.event);
        return this;
    };
    /* Convenience methods in jQuery namespace.           */
    /* Use as  $.belowthefold(element, {threshold : 100, container : window}) */
    $.belowthefold = function(element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).height() + $(window).scrollTop();
        } else {
            var fold = $(settings.container).offset().top + $(settings.container).height();
        }
        return fold <= $(element).offset().top - settings.threshold;
    };
    $.rightoffold = function(element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).width() + $(window).scrollLeft();
        } else {
            var fold = $(settings.container).offset().left + $(settings.container).width();
        }
        return fold <= $(element).offset().left - settings.threshold;
    };
    $.abovethetop = function(element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).scrollTop();
        } else {
            var fold = $(settings.container).offset().top;
        }
        return fold >= $(element).offset().top + settings.threshold + $(element).height();
    };
    $.leftofbegin = function(element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).scrollLeft();
        } else {
            var fold = $(settings.container).offset().left;
        }
        return fold >= $(element).offset().left + settings.threshold + $(element).width();
    };
    /* Custom selectors for your convenience.   */
    /* Use as $("img:below-the-fold").something() */
    $.extend($.expr[':'], {
                "below-the-fold" : "$.belowthefold(a, {threshold : 0, container: window})",
                "above-the-fold" : "!$.belowthefold(a, {threshold : 0, container: window})",
                "right-of-fold"  : "$.rightoffold(a, {threshold : 0, container: window})",
                "left-of-fold"   : "!$.rightoffold(a, {threshold : 0, container: window})"
            });
})(jQuery);